Math Problem Statement
Find out the sum of 2 + 12 + 36 + 80. up 20 terms.
Solution
The series given is . To find the sum of the first 20 terms, we need to identify the pattern in the series.
Step 1: Identify the pattern
Let's observe the pattern in the sequence:
The differences between consecutive terms (10, 24, 44) suggest that this is not an arithmetic sequence. Instead, the differences themselves seem to follow a pattern. Let's check the second differences:
Since the second differences are constant, this suggests that the sequence might be a quadratic sequence.
Step 2: Assume a quadratic formula
Assume the general term of the sequence is given by:
Using the first three terms to form equations:
- :
- :
- :
We now have the system of equations:
Step 3: Solve the system of equations
Subtract equation 1 from equation 2:
Subtract equation 2 from equation 3:
Subtract equation 4 from equation 5:
Substitute into Equation 4:
Substitute and into Equation 1:
Thus, the general term is:
Step 4: Find the sum of the first 20 terms
The sum of the first terms of the sequence is:
The sum of the squares, linear terms, and constants are:
Using the sum formulas: Substitute :
Thus:
So, the sum of the first 20 terms is .
If you need more details or have any questions, feel free to ask! Here are five related questions:
- How would the sum change if we extended the series to 30 terms?
- What if the sequence was geometric instead of quadratic? How would that change the sum calculation?
- Can you derive the sum formula for an arithmetic sequence?
- What would happen to the sum if the sequence started at a different term?
- How can you verify that the derived formula for is correct?
Tip: When identifying the pattern in a sequence, always check for differences between terms; if the first differences are not constant, check the second differences to see if the sequence is quadratic.
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Math Problem Analysis
Mathematical Concepts
Arithmetic Sequences
Quadratic Sequences
Summation Formulas
Formulas
Sum of squares formula
Sum of arithmetic series formula
Theorems
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Suitable Grade Level
Grades 10-12
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